This invention relates generally to sealed lead-in structures for electronic devices, and in particular to a glass-to-metal compression sealed lead-in structure, and to a novel technique for making such structures.
Sealed lead-in structures for electronic devices fall into two basic classes. In the first, called the "matched seal class," the lead-in conductor extends through the opening of an outer metal case, the annular space therebetween being filled with a glass seal. The metal of the case and that of the conductor have a like coefficient of thermal expansion (CTE). Typically, a matched seal structure makes use of a metal case and conductor, both composed of a low expansion cobalt-nickel-iron alloy and a glass seal of borosilicate glass having similar thermal expansion characteristics so that the unit is strain from (see Klebanoff et al. U.S. Pat. No. 2,826,630).
The present invention is concerned chiefly with the second class of lead-in sealed structures, commonly referred to as compression sealed. In this structure, seal integrity is imparted by compression stresses in the seal arising in the course of sealing (see Dalton U.S. Pat. No. 2,770,923). In order to achieve a compression seal, the outer case must have a high CTE, the lead-in conductor passing through the opening in the case must have a relatively low CTE and the glass seal must have a low or intermediate CTE.
Compression seal lead-in structures in which the outer case is composed of cold rolled steel and the inner conductor is formed of a nickel-iron alloy are widely used in the microelectronics industry to provide headers for transistors, integrated circuits and in various other applications.
Thus the Mayers U.S. Pat. No. 3,035,372 discloses a compression sealed glass-to-metal lead-in structure in which the case is SAE 1010 steel whose CTE is about 125.times.10.sup.-7 /.degree.C., the lead-in conductor is No. 52 nickel-iron alloy (about 51% nickel) whose average CTE is about 96-100.times.100.sup.-7 /.degree.C., and the seal is a glass composition whose average CTE is about 89.times.10.sup.-7 /.degree.C. It is also known, in lieu of alloy No. 52 for the lead-in conductor to use KOVAR (cobalt-nickel-iron) for this purpose.
The use of No. 52 alloy or KOVAR in a compression-sealed lead-in structure gives rise to certain problems. These alloys are not only relatively expensive but they are not readily available in all forms. Moreover, these alloys are very difficult to machine. Thus if the specification for a compression-type lead-in structure calls for a conductor of a non-standard gauge, or for threading or other details which entail machining, this requirement cannot be easily satisfied with existing alloys which are difficult to machine and it adds substantially to the cost of manufacturing the structure.
Another drawback in the use of No. 52 alloy, KOVAR or other iron and nickel containing alloys having ferromagnetic properties for the inner conductor of the lead-in structure is that when the structure is associated with an electronic component to which a high frequency current is applied through the conductor, then an electromagnetic field will be created that induces eddy currents in the conductor and generates heat that may be damaging to the seal or to the electronic circuit to which the conductor is connected.
The following additional patents are of background interest, for they deal with compression sealed glass-to-metal lead-in structures of various types: Earl--U.S. Pat. No. 3,545,950; Simpson--U.S. Pat. No. 4,128,697; Wildeboer--U.S. Pat. No. 3,356,466; Hodgdon et al.--U.S. Pat. No. 2,811,567; and Baas et al., U.S. Pat. No. 3,225,132.